Petroleum products are relatively volatile, and due to the nature of these products, storage tanks and the like must be vented to atmosphere to prevent build-up of excessive pressures in the tanks and also to provide a venting means during the filling of the tanks. Various of the products contained in these fuels are very volatile, particularly butane which is one of the major energy producing hydro-carbons in these fuels.
It is well known that evaporation takes place even in underground storage tanks, and this evaportion can be between 0.5% and 2.0% of the fuel, the rate depending upon the location and siting of the tanks and vent pipe, the factors affecting the evaporation rate being primarily climatic conditions such as temperature and winds, and also elevation above sea level.
Many vent pipes are fitted with what is termed an up-draft vent cowl which directs the vapours upwardly away from the surrounding area, but these up-draft vent cowls particularly where the cowl is situated in a windy location assist in the drawing out of the lighter vapours from the vent pipe and thus increase the evaporation rte.
Australian Pat. No. 511,957 discloses one form of valve arrangement intended for fitting to the vent pipe of a storage tank, but this has the disadvantage of the fact that during filling of the tank a back pressure exists which must be overcome during the filling operation.
My earlier Patent Application No. 29444/84 discloses an arrangement whereby this back pressure is overcome. However, there is still the problem that the vapours are not dispersed into the atmosphere in an effective manner.
It is an object of this invention to provide a valve arrangement on the vent pipe to reduce the loss of the vapours through the vent pipe while still maintaining the safety requirements of preventing the build-up of vapour pressure within the storage tank and to allow the passage of make-up air into the storage tank during withdrawal of fuel from the storage tank.
It is a further object of the invention to provide a vent cowl which assists in the upward extraction of the vapours and disperses these into the atmosphere.
It is a still further object to provide a self-cleaning filter to filter the air entering the tank during the withdrawal of liquid from the tank.